High voltage SMD capacitors and High power resistors. Do you use them ?

[Miyuki]

Hi fokls,

I need to place some snubbers.
"Problem is" 800V and 4W dissipated

There is 1kV rated 1206 Capacitors and 2W rated 2512 Resistors

Is it any problem or is it all fine to apply such stress to this tiny components (2512 is not so tiny I know) ?

[Mark]

Yes, they can be problematic.  Read the datasheets carefully. 

With high voltage SMT capacitors you might find mention of a conformal coating being necessary to meet the voltage rating. 
Even if coating is not necessary, bear in mind that handling (sweaty hands) and environment (damp) can increase the risk of arcing.  If an SMT cap arcs across the body it will need to be replaced because it will be much more likely to arc in the future. 

Also note that HV SMT caps can typically only withstand 20% overvoltage, where as leaded HV parts can normally take 200% rated (briefly). 

As for SMT high power resistors, check the power vs temperature derating curves carefully and add lots of copper to act as a heatsink, and possibly lots of vias to transfer heat to copper on other layers. 
Also check the pulse rating of the resistors. 

Mechanical stress is another thing to bear in mind, especially with the SMT caps, PCB flexing can cause cracking. 

[tszaboo]

Read up on X and Y capacitors, decide which one you want, if you need it at all.
For resistors, place many in series, that will reduce the power and voltage rating. It might be worth to place two in series, in case one would turn into charcoal.

[daqq]

We do use PULSED SMD power resistors for snubbers. You can get away with a LOT more power (~1+kW for a short pulse for a 2512 resistor designed for it). The average power on the resistors is relatively low (0.5W). See: http://www.farnell.com/datasheets/2140397.pdf , page 5.

Can't you use more resistors? Divide the power into a larger area, rather than using only one.

[Miyuki]

I can use 2-3 smaller ones, that is basically my question what will be better, in size, parasitic and reliability

I need to burn in resistor 800V ~1kW pulses with average power up to 4W
And capacitor to withstand up to 8A pulses

[T3sl4co1l]

1206 seems to be adequate for surprising voltages.  It's a ceramic body, so you don't have to worry about tracking.  You may want to rout a slot beneath it, though, because you do about the board!  2512 resistors should be okay as is.

Tim

[aiq25]

I have used 1206 ceramic capacitors before for a high voltage snubber (500V though). We had to conformally coat the PCBA to ensure voltage clearances were met.

Resistors should be ok.

[Neilm]

If I have a situation like this, I will always ask "is a snubber the only way of doing this?". Some years ago I had a situation where I had to dissipate about 1 W and had virtually no space to do it - the sub contractor had designed it then left. Only after I was tidying up the design did I spot the flyback transformer had no snubbing. I found a lossless snubber that didn't dissipate anything other than a few losses. As the thing was running off a battery it extended the battery life as well.

To answer the question, I have used both high voltage 1206 resistors and MELFs to disipate power. In those cases where the creepage on the PCB will be too small I put a small slot between the pads. It is a pain on 1206 but is doable.

[TimNJ]

At work, we use 1KV 1206s all the time, in AC-DC power supplies. What environment are you in? If you have relatively low humidity and low board contamination, you really don't need much to block 800V...Maybe 2mm creepage in most cases. Some of the IPC recommendations are pretty over the top. I *might* be a little concerned with thermal related stress of large SMD resistors, but at the same time, we use tons of 2512 power resistors...100,000s of products in the field and essentially no known failure modes related to over-stressed 2512s. Anecdotal evidence, but evidence nonetheless. Give them a little copper area to help spread the heat, parallel them, don't put them right on the edge of a board.